Spalling Tests Research Articles

Free Water Influence on the Dynamic Tensile Behaviour of Concrete

Concrete is a material used all over the world for civil engineering but the mechanisms governing its dynamic behaviour are still not well understood. In this work, spalling tests and edge-on impact experiments have been used to determine the influence of the free-water contained in pores and micro-cracks on the dynamic strength and on the fragmentation process. Moreover, spalling tests have been also used to identify the main mechanisms leading to the difference of behaviour observed between wet and dry concrete.

Effects of Two Different Dynamic Loading Conditions on Spall and Damage of 7075 Aluminum Alloy

The fracture behaviors of the 7075 aluminum alloy under two different dynamic loading conditions are investigated by means of a light-gas gun. The fracture surfaces obtained in the spall test are compared to the fracture surfaces obtained with a blunt projectile struck to the aluminum alloy plate. Optical and scanning electron microscopes are used in the investigation. For the plate-impact test, spall of the target was attributed to intergranular fracture caused by the tensile stress. The fracture behavior during projectile penetration is complex and consists of several fracture modes in addition to that the fracture is also of dynamic character. The penetration process of aluminum alloy target included: plugging stage, the microcracks nucleation stage, and the final tensile fracture stage. Mixed intergranular brittle/ductile fracture was observed, and brittle fracture played a dominate role.

Numerical Analysis of Concrete Material Properties at High Strain Rate Under Direct Tension

The tensile strength of concrete material increases with the strain rate. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, flexural test, spall test or splitting test (Brazilian test). Some codes of practice such as Comite Euro-International du Beton (CEB) give empirical relations of concrete material dynamic increase factor (DIF) based on testing data. However, the reliability of the dynamic testing and the derived DIF are under debating. It is commonly agreed now that the DIF obtained from dynamic impact test is affected by lateral inertia confinement effect. Therefore, those derived from testing data do not truly reflect the dynamic material properties. The influence of the lateral inertia confinement, however, is not quantified. Moreover, concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e. cement mortar only, in most previous experimental or numerical studies. In the present study, a mesoscale concrete material model consisting of cement mortar, aggregates and interfacial transition zone (ITZ) is developed to simulate direct tensile tests and to study the influences of the lateral inertia confinement and heterogeneity on tensile strength increment of concrete materials with respect to strain rates. The commercial software AUTODYN is used to perform the numerical simulations. The influence of lateral inertia confinement on tensile DIF of concrete material is examined.

Fatigue Failure and Thermal Spalling Tests to Evaluate Dynamic Fatigue Fracture of MgO–C Bricks

The relationship between compressive stress and loading cycles in MgO–C bricks with various carbon contents was investigated at room temperature and high temperature. The relationship between fatigue failure and thermal spalling was investigated by estimating a characteristic material constant, which represents the degree of sensitivity of crack growth, in terms of fracture mechanics. The relationship between the ratio of compressive stress for compressive strength and loading cycles in MgO–C bricks is not affected by the temperature of the atmosphere. At the same acting stress ratio, fatigue fracture life decreased as the carbon content in the MgO–C bricks decreased. In the thermal spalling test, the number of heating cycles at which small cracks and large cracks were generated decreased as the carbon content decreased. In the fatigue failure test, the ratio of the dynamic elastic modulus to the initial elastic modulus decreased gradually. This ratio also showed a gradual decrease in the thermal spalling test as the number of heating cycles increased. A characteristic material constant was obtained and compared based on the results of a fracture mechanics discussion. The characteristic material constant obtained from the fatigue failure test was substantially the same as that obtained from the thermal spalling test. From this, a relationship in which crack growth behavior leading to fatigue failure is equivalent to that of thermal spalling in MgO–C brick was recognized.

About the dynamic uniaxial tensile strength of concrete-like materials

Experimental methods for determining the tensile strength of concrete-like materials over a wide range of strain-rates from 10 −4 to 10 2 s −1 are examined in this paper. Experimental data based on these techniques show that the tensile strength increases apparently with strain-rate when the strain-rate is above a critical value of around 10 0–10 1 s −1. However, it is still not clear that whether the tensile strength enhancement of concrete-like materials with strain-rate is genuine (i.e. it can be attributed to only the strain-rate effect) or it involves “structural” effects such as inertia and stress triaxility effects. To clarify this argumentation, numerical analyses of direct dynamic tensile tests, dynamic splitting tests and spalling tests are performed by employing a hydrostatic-stress-dependent macroscopic model (K&C concrete model) without considering strain-rate effect. It is found that the predicted results from these three types of dynamic tensile tests do not show any strain-rate dependency, which indicates that the strain-rate enhancement of the tensile strength observed in dynamic tensile tests is a genuine material effect. A micro-mechanism model is developed to demonstrate that microcrack inertia is one of the mechanisms responsible for the increase of dynamic tensile strength with strain-rate observed in the dynamic tensile tests on concrete-like materials.

Influence of aggregate size and free water on the dynamic behaviour of concrete subjected to impact loading

Concrete is a material widely used in civil engineering. Thus the knowledge of its mech anical behaviour is a major safety issue to evaluate the ability of a stru cture to resist to an intense dynamic loading. In this study, two experimental techniques have been applied to a m icro-concrete and a co mmon concrete to assess the influence of the aggregate size on the dynamic response. First, spalling tests o n dry and wet specimens have been performed to characterize the tensile strength of concrete at strain rates in the range 30 - 150/ s. T hen, e dge-on i mpact t ests i n s arcophagus c onfiguration ha ve be en conducted. The cracking pattern of the micro-concrete and the concrete plates in wet and dry conditions have been compared to appraise the influence of aggregate size and free water on the damaging process.

Test method for concrete spalling using small electric furnace

AbstractConcrete spalling can cause severe damage to concrete structure when exposed to fire. The spalling mechanisms are not very well understood. For the testing of spalling, full‐scale structural members should be used, as spalling tests are sensitive to size effects. Full‐scale testing in large furnace is costly and is not suitable for testing large number of concrete mixture trials. The standard and hydrocarbon fire time–temperature curves have rapid temperature rise during the initial phase. This temperature rise requires a gas furnace with high heating capacity and cannot be generated by electric muffle furnace commonly available in many laboratories.This paper presents a method to carry out spalling test in small‐scale specimens with exposure to rapid temperature rise using a commonly available electric furnace in the laboratories. The tests are based on 150 mm diameter cylinders that are laterally confined to simulate full‐scale structural members. The cylinder surface is exposed to rapid temperature rise by exposing through vertical and/or horizontal holes in pre‐heated small electric furnace. Some unconfined 100 mm diameter cylinders were also exposed horizontally to test the performance of confinement. The paper shows that the hydrocarbon fire and standard fire exposure can be simulated by manipulating the exposure location of the surface of the concrete cylinder. Ordinary Portland cement concrete cylinders with different strengths were tested and different spalling patterns were observed. The spalling patterns matched the test results from a gas furnace fire test simulating the fire curves. The tests demonstrated that the method is an effective and convenient technique to predict the spalling risk of a concrete. Copyright © 2009 John Wiley & Sons, Ltd.

Dynamic fragmentation process in concrete under impact and spalling tests

Intense damages as scabbing on front face, spalling on rear face, radial cracks are observed in concrete structures when subjected to the impact of a kinetic striker. To characterize the dynamic strength and damage of concretes under such loadings one may perform spalling tests and EOI (edge-on impact) tests. Both tests have been conducted with dry and wet specimens of a micro-concrete named MB50. The tests revealed a remarked effect of strain-rate and free water on the dynamic response of the concrete. In parallel, bending tests and direct tensile tests have been performed with dry and saturated concrete samples considering large and small effective volumes and the results have been compared with five sets of data given in the literature with the same material. A scale effect is observed in agreement with prediction of Weibull theory. Moreover, an anisotropic damage model that describes the initiation of cracks and the obscuration of critical defects under high strain-rate tensile loading is presented. It allows accounting for the influence of loading-rate and free-water on the dynamic strength and damage of concrete observed in EOI and spalling tests.

An Experimental Method to Determine the Tensile Strength of Concrete at High Rates of Strain

In the present work, dynamic tensile strength of concrete is experimentally investigated by means of spalling tests. Based on extensive numerical simulations, the paper presents several advances to improve the processing of spalling tests. The striker is designed to get a more uniform tensile stress field in the specimen. Three methods proposed in the literature to deduce the dynamic strength of the specimen are discussed as well as the use of strain gauges and a laser extensometer. The experimental method is applied to process data of several tests performed on wet micro-concrete at strain rates varying from 30 to 150/s. A significant increase of the dynamic tensile strength with strain-rate is observed and compared with data of the literature. In addition, post-mortem studies of specimens are carried to improve the analysis of damage during spalling tests.

Boundary effect on weight function in nonlocal damage model

Some insights on boundary effects in nonlocal damage modelling are addressed. Interaction stresses that are at the origin of nonlocality are expected to vanish at the boundary of a solid, in the normal direction to this boundary. Existing models do not account for such an effect. We introduce tentative modifications of the classical nonlocal damage model aimed at accounting for this boundary layer effect in a continuum modelling setting. Computations show that some nonnegligible differences may be observed between the classical and modified formulations. In a one dimensional spalling test, only the modified formulation provides a spall of finite nonzero thickness, whereas spalls smaller than the internal length cannot be obtained according to the original formulation. For the same set of model parameters, including the internal length, the fracture energy derived from the size effect test method is also very different according to both approaches. Parameters in the size effect laws for notched and unnotched specimens, obtained from computation of geometrically similar bending beams, are more consistent with the modified nonlocal model compared to the original nonlocal formulation.

Behavior of Densified Normal Strength Concrete under Elevated Temperature

This paper presents an experimental investigation on fire resistance of densified normal strength concrete (DNSC), at water/binder (W/B) ratios of 0.45, 0.36, and 0.32, of which compressive strength of 28-days ranged from 42.5 MPa to 56.3 MPa. The results of the spalling test reveal that DNSC encountered explosive under high temperature. Polymer fiber can be used to improve fire resistance of DNSC. DNSC subjected to high temperature lost its mechanical properties in a similar manner to that of high-strength concrete.

Validating Theories for Brittle Damage

Validating simulated predictions of internal damage within armor ceramics is preferable to simply assessing a model’s ability to predict penetration depth, especially if one hopes to perform subsequent “second strike” analyses. We present the results of a study in which crack networks are seeded by using a statistically perturbed strength, the median of which is inherited from a deterministic “smeared damage” model, with adjustments to reflect experimentally established size effects. This minor alteration of an otherwise conventional damage model noticeably mitigates mesh dependencies and, at virtually no computational cost, produces far more realistic cracking patterns that are well suited for validation against X-ray computed tomography (XCT) images of internal damage patterns. For Brazilian, spall, and indentation tests, simulations share qualitative features with externally visible damage. However, the need for more stringent quantitative validation, software quality testing, and subsurface XCT validation, is emphasized.

Void growth by dislocation-loop emission

Experimental results from spall tests on aluminum reveal the presence of a dense dislocation structure in an annulus around a void that grew under the tensile pulse when a shock wave was reflected at the free surface of the specimen. The proposition is that dislocation emission from the void surface under load is a viable mechanism for void growth. To understand void growth in the absence of diffusive effects, the interstitial-loop emission mechanism under tensile hydrostatic stress is investigated. First, the micromechanics of pile-up formation when interstitial loops are emitted from a void under applied macroscopic loading is reviewed. Demand for surface energy expenditure upon void-surface change is taken into consideration. It is demonstrated that in face-centered cubic metals loop emission from voids with a radius of ∼10 nm is indeed energetically possible in the hydrostatic stress environment generated by shock loading. On the other hand, the levels of hydrostatic stress prevalent in common structural applications are not sufficient to drive loops at equilibrium positions above a ∼10 nm void. However, for voids larger than about 100 nm, the energetics of loop emission are easily met as a necessary condition even under the low stress environment prevalent in structural applications.

Study on constitutive relations and spall models for oxygen-free high-conductivity copper under planar shock tests

It is indicated that the constitutive relations for shear modulus G as functions of thermodynamic state were not given in the Johnson-Cook model and the Zerilli-Armstrong model while the constitutive relations for yield strength Y as functions of strain and strain rate were not given in the Wallace model and the Straub model. Seven constitutive models are constructed based on Y∕G=constant and on G∕B=constant (B-bulk modulus), respectively. The variations of longitudinal stress, transverse stress, and yield strength of oxygen-free high-conductive (OFHC) copper with time under planar shock loading are obtained using the manganin stress gauges and compared with the predicted results by the constructed seven constitutive models. It seems that the pressure, density, temperature, and plastic strain dependence of the yield strength for OFHC copper subjected to planar shock loading is essential to the constitutive description. Planar spall tests for OFHC copper are also performed and the void coalescence-based spall model presented by the authors are applied to numerical simulations of planar spall tests including tests given by Seaman et al. [Phys. Rep. 147, 253 (1987)]. The Steinberg-Cochran-Guinan constitutive model is proved appropriate to the planar shock loading and is adopted in the numerical simulations of planar spall tests.

Spall investigations for LY12 Al using triangular waves

Spall of LY12 Al was investigated using experimental techniques based on the dynamics of shock wave attenuation to produce decaying triangular shock pulse in the sample of plate-impact spall tests. Spall signals were measured by monitoring the time-resolved free surface velocity histories of the targets with VISAR techniques. Targets were soft recovered and two spall planes for the high-stress triangular wave experiment were observed. For triangular wave spall plane location is a variable and there can be several regions of relatively high tension. The void coalescence-based spall model presented by the authors is used in simulating the spall tests. Computed free surface velocity histories of targets and damage distributions through the thickness of the targets are compared with the VISAR data and the observed damages in soft recovered targets, respectively. It is noted that the modeling of spall process caused by triangular waves can be seriously influenced by the artificial viscosity, the constitutive equations of the sample and the spall fracture model.

Effect of Rare Earth on Void Band of Diffusion Layer and Properties of Aluminized Steel

Abstract The effects of the addition of rare earth (RE) elements on the void band in the diffusion layer, and the resistances to both oxidation and spalling of aluminized steel were investigated through high temperature oxidation and spalling tests. The results showed that RE had significant effects on the void band in the diffusion layer and the properties of aluminized steel. After diffusion treatment, a considerable number of the voids between the middle layer and transitional layer of pure aluminized coating, aggregated into wavy-line-shaped void bands parallel to the outer surface. For the RE-added aluminized coating, only a few voids aggregated into intermittent block shapes. During high temperature oxidation at 800 °C for 200 h, the wavy void band of pure aluminized coating aggregated further into a linear crack parallel to the outer surface, and the internal oxidation occurred within them; the open cracks perpendicular to the surface penetrated through the diffusion layer. For the RE-added aluminized coating, only a few voids aggregated into intermittent meniscus shapes. During cyclic spalling tests, the peeling, spallation, and pulverulent cracking occurred along the void band in the diffusion layer of pure aluminized coating, but only a little spallation occurred in the diffusion layer of the RE-added aluminized coating, in which cracks perpendicular to the surface were much smaller than those of pure aluminized coating and did not penetrate through the diffusion layer. It is evident that RE addition can restrain the formation and aggregation of voids and subsequently improve the resistances to oxidation and spalling. The mechanism of the RE effect on the void band in the diffusion layer is also discussed.

Tensile failure of concrete at high loading rates: New test data on strength and fracture energy from instrumented spalling tests

For the numerical prediction of the response of concrete structures under extreme dynamic loading, like debris impact and explosions, reliable material data and material models are essential. TNO-PML and the Delft University of Technology collaborate in the field of impact dynamics and concrete modelling. Recently, TNO-PML developed an alternative Split Hopkinson Bar test methodology which is based on the old principle of spalling, but equipped with up-to-date diagnostic tools and to be combined with advanced numerical simulations. Data on dynamic tensile strength and, most important, on fracture energy at loading rates up to 1000 GPa/s are obtained. The paper describes the test and measurement set-up, presents the new test data and the analysis of the test results. In addition, a rate-dependent softening curve is given which is based on the integrated findings so far.

Glass damage by impact spallation

This paper presents an investigation of spallation in impact-loaded glass. For this purpose, spall tests on cylindrical specimens were performed for various impact velocities using a Hopkinson bar “HB”. After each test, the specimen was rebuilt from restored fragments and the analysis of the fracture surface was performed by means of a scanning electron microscope. Fracture is found to propagate in a transversal way. In addition, numerical modeling allowed an appropriate prediction of the experimental stress level, time to fracture and fracture location. Based on microscopic observations of the fractured surface, a new fracture criterion has been proposed. It defines the damage variable as a function of the ratio between the initial surface and its evolution.

Elastic wave propagation in cylindrical bars after brittle failures: Application to spalling tests

The effect of brittle failures on the elastic wave propagation along cylindrical bars is analysed. From experimental observations provided by spalling tests of ceramic materials, a theoretical analysis is carried out based on a finite elements simulation of the experiments and a mathematical analysis of the pulses by means of Fourier Transform techniques. Differences between the propagating waves before and after the material failure are revealed. After failure, the pulse is influenced by dispersion effects and its shape changes during propagation. To correct this effect, a procedure based on Bancroft's curves is suggested. Finally, some clues about the way to get consistent results from spalling tests are given.

A void coalescence-based spall model

A void coalescence-based spall model is presented using stress relaxation equations based on the assumption that the main effect of the microcracks is to reduce the area over which the stress acts in the early stages and the stress decreases to porosity-dependent value in the void coalescence stages. The stress- (or pressure-) dependent spall porosities given by Thomason, by Tonks et al and by Cochran et al. are, respectively, combined with conservation equations, equation of state and constitutive equations for the damaged aggregate to establish a series of closed equations for all variables including the damage. The void coalescence-based spall model contains only two parameters: the spall strength and critical damage, the values of which can be initially estimated for plate-impact spall tests and be finally determined to make the computed results of spall tests under the initial and boundary conditions consistent with experimental velocity (stress) profile and the observed damage at spall plane in general. The computer simulations of spall experiments for copper, uranium and steel are performed with the one-dimensional Lagrangian finite difference method. The computed results based on the pressure-dependent spall porosities given by Tonks et al., and by Cochran et al., are consistent in general, but different from the computed results based on Thomason's 2D stress-dependent spall porosity to a considerable extent.